1. Field of the Invention
The present invention relates to a method of manufacturing multilayer molded articles of at least two plastics material components having substantially different properties with respect to behavior, processing and/or temperature changes. The method includes injecting the plastics material components from different extruder units through a common sprue bushing into the mold cavity of the molding tool.
The present invention further relates to an apparatus for carrying out this method.
2. Description of the Related Art
A method of the above-described type and an apparatus for carrying out the method are known from German Offenlegungsschrift No. 27 05 291. In this method, molded articles of thermoplastic material having a solid outer skin and a porous core are manufactured. The method includes conducting the plastics material component for the solid outer skin and the plastics material component containing an expanding agent through separate ducts into the sprue bushing and bringing the two components into contact only in this sprue bushing, preferably near the outlet side of the sprue bushing.
In the apparatus for carrying out the method, the sprue bushing has at its inlet side at least two separately arranged flow ducts which are each connected to an extruder unit and lead into a common injection duct.
However, the known apparatus has the disadvantage that the sprue inevitably formed when the molded articles are manufactured has toward the molded article a relatively large cross-sectional size which can only be cleanly removed from the finished molded article with the aid of complicated additional apparatus.
The known apparatus has the additional disadvantage that the approximately Y-shaped sprue bushing is difficult to manufacture and, therefore, is relatively expensive.
EP-A-0 197 830 discloses a method of the above-described type for manufacturing multilayer molded articles in which the skin material is a thermoplastic material while the core material is a duroplastic material.
This known method also has the disadvantage that the sprue inevitably created during injection molding of the molded article has a substantial cross-sectional area at the connection to the molded article and, therefore, the sprue can only be separated from the molded article with substantial additional work.
The apparatus for carrying this method has the further disadvantage that it is only possible to use one nozzle head which has at least two concentric orifices, i.e., a central orifice and an annular orifice surrounding the central orifice surrounding the central orifice.
If the known apparatus is to be used for carrying out the method with different plastics material components having different temperatures, it is a necessary requirement that each of the nozzle ducts or passage ducts which are utilized has its own heating system and/or cooling system. However, the simultaneous processing of plastics material components having different temperature levels is still very difficult because these plastics material components flow together into concentric material flows within the sprue bushing which has a relatively large volume. This has the inevitable undesired result that the temperature levels of the different materials approach each other which may significantly impair the quality of the finished molded article. For example, the manufacture of multilayer molded articles of soft PVC and/or polycarbonate is very problematic. Moreover, it is extremely difficult or even impossible to use the last-described known method for processing thermally sensitive plastics materials.
It is, therefore, the primary object of the present invention to provide a method and an apparatus for manufacturing multilayer molded articles in which the disadvantages of the above-described known methods and apparatus are avoided.
More specifically, it is an object of the present invention to provide a method and an apparatus for carrying out the method of the above-described type which make it possible to manufacture multilayer molded articles of substantially different plastics materials, such as, thermoplastic materials, duroplastic materials, elastomers, and thermally sensitive plastics materials, such as hard PVC, without problems and in a safe manner, wherein the sprues toward the molded article which inevitably are formed during the injection process have a very small cross-sectional size.